1. Field of the Invention
This invention relates to sag bending of glass sheets in a bending lehr and in particular to a method and apparatus to reduce tip curl caused by sag bending complicated shapes in glass sheets.
2a. Technical Considerations
The techniques of sag bending to form shaped glass windows for automobiles and the like, as disclosed in U.S. Pat. No. 4,375,978 to Reese, is well known. Glass sheets are positioned on and supported by a skeletal bending mold. The shaping rail of the mold conforms to the shape and final configuration of the glass sheet to be shaped, slightly inboard of the edge of the glass. The bending molds are then conveyed in succession through a heating lehr where the glass is heated to its deformation temperature and begins to sag by gravity until the glass sheet conforms to the configuration of the shaping rail. After the glass sheets have properly deformed, the molds are conveyed through an annealing zone where the glass sheets are cooled, in a controlled manner, from their deformation temperature through their annealing range to anneal the glass sheets.
The glass sagging technique has been the method used to bend two glass sheets simultaneously, which sheets are subsequently laminated together to form a laminated automobile windshield. The windshield is curved to conform and blend into the shape of an automobile vehicle in which it is installed.
In recent years, automobile stylists have demanded more complex and deeper bends in glass doublets used in windshields. When glass sheets are shaped to relatively deep bends, the length of the chord between the ends of the bent glass is shorter than the distance between the ends of the flat glass sheet before bending. In order to reduce relative sliding between the glass ends and the shaping rail which marks the glass, the mold rail comprises articulated rail sections including an end rail section whose contour conforms to the outline and shape desired for the longitudinal end portion of the glass sheet to be bent. The end rail section is constructed and counterweighted to pivot downward into a lower position to support the mass of a relatively stiff flat glass sheet to be bent, and when the glass sheet is softened by heat, to pivot upward into an upper position where it cooperates with additional shaping rails to form a substantially continuous outline shaping surface conforming in contour and elevation to the shape desired for the glass sheet. When the desired final configuration is particularly complex such as required very deep bends and/or reverse curvatures, the heat pattern within the lehr can be adjusted to direct localized heat toward these critical bend areas.
It has been found that with complex deep bends, the outermost portions of the longitudinal end portions, or tips, of the glass sheets tend to curl and lift off portions of the end rail section causing the sheets to deviate from desired tolerances. This tip curl is caused by the tendency of the sheets to draw glass from its end sections when there is a deep sag or a reverse curve. It can also be caused by overheating the tip areas to achieve proper curvature along the outer edge of the longitudinal section, or A post, of the glass sheet. The local heat tends to preheat the glass at the tip area causing it to rise off the end rail section. The tip does not get a chance to relax and sag back onto the end rail.
It would be advantageous to develop a method of forming glass sheets to the new complex and deep sag configurations while maintaining the desired outline configuration.
2b. Patents of Interest
U.S. Pat. Nos. 4,015,969 to Brown et al. and 4,077,791 to Oelke teach an apparatus for bending glass to a relatively sharp angle. A contact assembly is positioned over the edge of the glass sheet and engages a conductor element on a surface of the sheet throughout the bending operation. The contact assembly is held in positive engagement with the surface of the sheet by a spring mechanism so that contact is maintained despite vertical translation of the conductor elements. When the sheet is fully bent, contact assemblies must be rotated off the surface of the glass to allow its removal.
U.S. Pat. No. 4,173,461 to Ebata et al. teaches the bending of hot glass sheets wherein the edge of the sheet is positively clamped to the top of a bending table to maintain one edge of the sheet against the table during bending. The glass is heated to its deformation temperature and droops on account of its own dead weight onto the table where it assumes the contour and configuration of the top. The clamped edge of the sheet remains in contact with the table throughout the bending operation and the clamps must be removed in order to remove the shaped glass.
U.S. Pat. No. 4,193,785 to Bailey teaches the use of opposed locator stops on a glass bending mold. The stops contact the edge of the glass sheet to be bent and exert pressure on the ends of the sheet while maintaining the sheet in proper relation to the mold throughout the bending operation. The locator stops contact only the edge of the glass and do not provide a means for maintaining positive contact of the glass sheet against the bending mold.